1. Field of the Invention
The present invention relates to an instrument and method for shaping a femur preparatory to implanting a knee prosthesis.
2. Description of Related Art
A total knee replacement or prosthesis, substitutes for a patient""s arthritic or otherwise dysfunctional natural knee joint. The prosthesis offers the patient an alternative treatment for the chronic pain and discomfort often associated with such problems.
Burstein et al. U.S. Pat. No. 4,298,992 illustrates a popular total knee joint prosthesis known as the Insall-Burstein (I/B) knee. Such prostheses comprise a femoral component attached to the patient""s femur, a tibial component attached to the patient""s tibia, and a patellar component attached to the patient""s patella. To use this prosthesis, it is necessary to shape by resection the patient""s femur, tibia and patella. The tibia and patella are shaped by a flat cut. In the case of the I/B prosthesis, the femoral component requires five cuts about the periphery of the femur and three cuts in the intercondylar notch in the distal end of the femur. These cuts conform to complementary portions of the femoral component which engage the bone. It is important that these femoral cuts be made precisely and located so that the position of the femoral component and the tibial component will closely approximate the positions of their anatomical counterparts. Proper positioning of the femoral and tibial components is required to insure the patient""s natural anatomic limb alignment and normal anatomical movement.
As part of the installation of a knee joint prosthesis, it is important to balance the ligaments crossing the joint. Balancing the ligaments means performing soft tissue releases, as necessary, so as to ensure that when the femur is lifted off of the tibia by a force directed along the long axis of the tibia, and the tibia is positioned in natural anatomic limb alignment, the tension in the medial and lateral ligamentous support is nearly equal. In order for the ligamentous tension to be nearly equal, the distance between the medial aspect of the femur and tibia, and the lateral aspect of the femur and tibia must be nearly equal when the tibia is in natural anatomic limb alignment. The distance between the femur and tibia should also be approximately equal throughout the range-of-motion of the tibia from full extension to approximately 90 degrees of flexion. The distance between the femur and tibia is commonly called xe2x80x9cthe gapxe2x80x9d, and the process of lifting the femur, and measuring the distance between the femur and tibia is commonly called xe2x80x9cchecking the gapxe2x80x9d. Heretofore, gap checking has been done after the femur has been shaped and only at full extension, and 90 degrees flexion. If proper balancing can not be achieved, then it may be necessary to reshape the femur, which is obviously undesirable, or to use a more constrained total knee design, thus limiting knee motion.
An object of the invention is to provide an instrument for use in shaping the distal surface of a femur preparatory to implantation of a knee prosthesis in which the gaps can be checked and adjusted prior to shaping the femur. This eliminates the need to recut the femur and reduces the time required for surgery. Moreover, since it is easier to check the gaps with the invention, it is likely that more surgeons will take the time to check the gaps and, therefore, the quality of the surgery will improve.
A further object of the invention is to provide a device for shaping the distal surface of a femur preparatory to implanting a knee prosthesis wherein all of the cuts required to shape the femur can be made using a single instrument.
Yet another object of the present invention, is the ability to check the gaps continuously throughout the range-of-motion of the knee, not just at full extension and 90 degrees flexion, as with conventional devices.
The most popular types of instruments used to shape the femur are intramedullary devices in which a cutting guide is positioned at the distal end of the femur relative to a rod which is driven into the intramedullary canal of the femur. Historically, surgeons have also used extramedullary alignment to orient the instruments to cut the distal femur. The present invention may be used with intramedullary or extramedullary alignment techniques.
The principal component of the knee instrument according to the invention comprises a gap checking device which includes a curved base adapted to engage the distal end of the patient""s femur. A plurality of slots are defined in the gap checking device for guiding the cutting instrument used to shape the femur. The gap checking device can be referenced to the patient""s femur using conventional intramedullary or extramedullary locator techniques.
The thickness of the gap checking device is such that the distance between the distal cut in the femur to the distal surface of the gap checking device plus the thickness of a shim corresponds in a predetermined way to the thickness of the femoral and tibial components of the prosthesis. Accordingly, the gaps can be checked with the gap checking device after placing the shim on the cut surface of the tibia. The shim also protects the cut surface of the tibia to prevent bone damage. After the knee has been properly balanced, bone cuts are made by passing a cutting instrument through the slots in the gap checking device. If necessary, a slot extender may be attached to the gap checking device to extend or increase the length of the slots.